M33 is a nearby example of a spiral galaxy. It is only slightly inclined
to our line of sight such that we can see both structure in the spiral
arms, and the central bulge. Observations of galaxies like these can help
us determine the structure of our own Galaxy, the Milky Way.

This image shows M33 in X-rays as observed by the Einstein
Observatory, a telescope which orbited the Earth between 1978 and
1981. The background greyscale image is an optical photograph while the
contours are the X-ray data. This shows that most of the high energy
photons are coming from the centre of the galaxy where astronomers think
that a black hole exists.